Many parts (heat generating parts) that generate heat when they are operated are used for a projection type display device. Thus, a general projection type display device is provided with a cooling means for supplying the heat generating parts with cooling air. Specifically, the projection type display device is provided with a fan for taking air in and a channel for introducing the air taken in by the fan into the heat generating parts or their vicinity. Further, in order to keep out dust, an air filter is arranged at an air taking-in port and the channel.
However, when the projection type display device is used in a dusty environment or is used for a long time, the air filter is clogged. When the air filter is clogged, the heat generating part cannot be supplied with sufficient cooling air and hence the temperature of the part is increased. When the temperature of the part is increased, the operation of the part is made unstable or the life of the part is made short. Thus, detecting the clogging of the air filter at an early stage and with reliability is very important for maintaining the performance of the projection type display device and for improving the reliability of the projection type display device.
For example, in JP06-221599 is disclosed a projection type display device which detects the clogging of an air filter on the basis of the result of a comparison of temperature (T1) detected by a thermistor arranged near a liquid crystal light valve and temperature (T2) detected by a thermistor arranged near the air filter (related art 1). When the clogging of the air filter is caused, temperature near the liquid crystal light valve is increased, so that the clogging of the air filter can be detected by comparing temperature (T1) with temperature (T2).
In JP2001-252513 is disclosed a dust detection device which detects the clogging of an air filter on the basis of the result of comparison of pressure (P1) detected by a pressure sensor arranged outside a housing and pressure (P2) detected by a pressure sensor arranged inside the housing (related art 2).
In JP2005-17547 is disclosed an image display device that has a light emitting section and a light receiving section arranged across an air filter and that detects the clogging of an air filter on the basis of a change in the amount of light received by the light receiving section (related art 3). When the air filter is clogged, the amount of light passing through the air filter is decreased, so that the clogging of the air filter can be detected on the basis of a change in the amount of light received.
In JP09-155134 is disclosed a filter winding control device which detects the clogging of an air filter on the basis of a change in the amount of the load of a fan motor (related art 4).